The objective of this program is to develop an improved high dose rate brachytherapy system for the treatment of cancer based on the low-energy gamma ray source 169Ytterbium. Such development would achieve the benefits of superior dose optimization with its collateral improvement in therapeutic response, along with potentially better treatment efficacy available with afterloaded high dose rate temporary brachytherapy sources. The program will demonstrate that the reduced shielding requirements for this radionuclide can be translated into more conformal dose distributions to the treatment target, reduction of collateral dose to the patient and elimination of the high capital cost of a dedicated, shielded treatment room such as is required for the currently-used 192Iridium source. We aim to demonstrate both clinical and economic benefits for the proposed method over currently practiced high dose rate brachytherapy treatment using 192Iridium. [unreadable] [unreadable] In addition to a low-energy/high activity source of 169Ytterbium, we intend to develop shielded treatment devices to improve dose conformity and collateral local shielding to permit the performance of HDR brachytherapy in minimally shielded treatment room. This system will remove the barrier of a large capital investment. We anticipate that this development will extend the use of high dose rate brachytherapy to a much larger number of clinical centers, partly because with this new source, intraoperative treatments would be physically possible in regular (i.e. unshielded) operating rooms. With the increased use of HDR brachytherapy for the treatment of prostate cancer, and with the recent enthusiasm about the treatment of breast cancer using this technique, this development is particularly important at this time. [unreadable] [unreadable] [unreadable]